Etching and cleaning of SiO.sub.2, tungsten, polysilicon and Si.sub.3 N.sub.4 materials are technologically significant processes for microelectronics device fabrication. Faster etch rates are required to increase the throughput of the process, thus decreasing the total cost.
In most etching processes, diluting the fluorine source gas, CF.sub.4, C.sub.2 F.sub.6, etc. results in a decrease in SiO.sub.2 or Si.sub.3 N.sub.4 etch rates. The NF.sub.3 literature reports the same result, diluting the NF.sub.3 results in lower etch rates.
Various attempts have been made to use nitrogen trifluoride efficiently in the prior art.
U.S. Pat. No. 4,711,698 describes a plasma etching process for thermally grown or CVD deposited silicon oxide. This work describes selective etching of silicon oxide to silicon and refractory metals and their silicides. The preferred embodiment uses a gas mixture of BF.sub.3 /H.sub.2 /Ar with the claim that the BF.sub.3 gas can be replaced by one of the following: NF.sub.3, SF.sub.6, or SiF.sub.4 Hydrogen is supplied to the reactor in a carrier gas of argon in the concentration of 3% hydrogen to 97% argon. Total flow rates are calculated from the flow rate of the fluorinated gas (BF.sub.3, etc.) and the hydrogen flow rate, with the inert gas component disregarded. The inventor claims the percentage in the etchant gas mixture of all atomic species which form in the glow discharge molecules and radicals capable of adsorbing onto and reducing silicon oxides in the range of 5% atomic to 50% atomic of the etchant gas mixture. The pressure range claimed is 100 mTorr to 3 Torr. Also claimed is the atomic ratio of fluorine to hydrogen in the etchant gas mixture in the range of 15:1 to 30:1.
Japanese Unexamined Patent Application 63-11674 is an improved cleaning method for plasma CVD chambers utilizing mixtures of argon and NF.sub.3. In particular, Si.sub.3 N.sub.4 was etched with a 50% mixture (nitrogen trifluoride/argon) at 0.6 Torr. Data shown in this patent application range from 0.3 to 1.0 Torr with 50% dilution.
H. G. Stenger and G. S. Akiki, "Kinetics of Plasma Etching Silicon with Nitrogen Trifluoride," Mat. Res. Soc. Proc. Vol. 68, (1986) pp. 267-272, state that, "A potential drawback from the use of NF3 is its formation of fewer positive ions than CF4 at similar reaction conditions [2]. Fewer positive ions cause NF3 to give predominately isotropic etch profiles [4]." Etching was performed in a Plasma Therm PK-24 radial flow reactor with electrode spacing of 1.57 cm and electrode area of 2450 sq cm (55.8 cm dia.). The reactor pressure was held at 18.7 Pa, electrode temperature was held at 25.degree. C., and mole fraction of NF.sub.3 in Ar held at 0.4. Etch rates of silicon (given in micromoles of Si/min) were a linear function of inlet gas flow (from 4 to 24 sccm) with power densities of 0.082, 0.123, 0.163, and 0.204 W/cm.sup.2. "As silicon loading is decreased, loss of fluorine atoms in non-etching reactions will become significant relative to those consumed in the etching reaction."
J. Barkanic, A. Hoff, J. Stach, and B. Golja, "Dry Etching Using NF3/Ar and NF3/He Plasmas," Semiconductor Processing, ASTM STP 850, (1984) pp. 110-123, include research in which experiments were performed in a Plasma Therm PK 2440 Dual Plasma/Reactive Ion Etch System with 22 in Dia. electrodes and electrode spacing of 2.6 inches. Helium and argon were used as diluents with the NF.sub.3 being varied from 10% to 80% of the total flow (by volume). For plasma etching (PE) experiments a fixed flow of 40 sccm was used and pressure ranged from 60 to 500 microns. For RIE the flow was varied from 10 to 40 sccm and the pressure ranged from 15 to 80 microns. Power densities ranged from 0.02 to 0.8 W/cm.sup.2. "At low NF3 concentrations (&lt;40% NF3/inert) the etch rate in either an Ar or He mixture is not significantly different. In addition, the etch rate for low percent NF.sub.3 mixtures (10 to 20% NF3/inert) doesn't vary markedly with power density. This indicates that the NF3 concentration in low percentage NF3 mixtures is low enough such that it doesn't result in more NF3 being dissociated as power density is increased." SiO.sub.2 etch rates varied from 30 to 1500 A/min depending upon the mode selected. Loading of the chamber was also noted to have an effect on etch rate with the rate decreasing with increasing number of wafers.
V. M. Donnelly, D. L. Flamm, W. C. Dautremont-Smith, and D. J. Werder, "Anisotropic Etching of SiO.sub.2 in Low-Frequency CF4/O.sub.2 and NF3/Ar Plasmas," J. Appl. Phys., vol. 55, no. 1, (Jan. 1984), pp. 242-252, describe a protocol in which during all experiments gas pressure was 0.35 Torr and total flow rate was 100 sccm. Additionally, three types of electrodes were used: (1) hard anodized aluminum; (2) Stainless steel; and (3) silicon covered stainless steel lower electrodes. "NF3/Ar plasmas generated much higher fluorine atom concentrations than CF4/O.sub.2 plasmas . . . The fluorine atom density is .about.10 times higher than in CF4/50% O.sub.2 under the same conditions (empty stainless-steel reactor, power =0.35 W/cm.sup.2, flow rate =100 sccm, and pressure =0.35 Torr)." When the lower electrode was covered with silicon the fluorine atom concentration in the NF.sub.3 plasma dropped by a factor of .about. 6. Their results of etch rate vs. fluorine atom concentration suggest that the same processes are operative in both CF.sub.4 /O.sub.2 and NF.sub.3 /Ar for SiO.sub.2 etching. Additionally, it was found that the higher the percentage of NF.sub.3 in the mix, the higher the resulting substrate temperature. Contamination was found to be "less serious" when the stainless steel electrodes were used.
S. M. Tan, H. C. Goh, H. A. Naseem, and W. D. Brown, "Plasma Etching of Silicon Using NF3 Diluted with Argon, Nitrogen, and Hydrogen," Proc. 2nd Int'l. Conf. on Elec. Mats., (1990 Materials Research Society), pp. 439-444, reported that experiments were performed in a single chamber stainless steel PECVD system. The chamber pressure was kept at 250 mTorr, the total flow rate was 10 sccm, and the power was varied from 4 to 20 Watts (0.023 to 0.113 W/cm.sup.2). Etch rates were measured by profilometry. It was observed that the sustaining power for NF.sub.3 diluted with argon or hydrogen was about half that for dilution with nitrogen. The authors point out that their etch rate data is in contrast to that of Barkanic (i.e., the etch rates of silicon vs. dilution % peak at different concentrations, with power held constant, and are not linear as in the Barkanic studies). They explain this result by stating ". . . although there is a higher concentration of NF3, the concentration of reactive etching radicals seems to decrease, resulting in a lower etch rate. With constant power applied to the plasma, the fall in the etch rate at higher R [R=NF3 flow rate/(NF3+diluent gas)] can be observed as the operating power for etching gets nearer to the plasma sustaining power. Increasing the concentration further would put the power in the no plasma region . . ." That is, as the concentration of NF.sub.3 increases it becomes harder to "light" the plasma, i.e., more power is needed to sustain the plasma.
In their study of NF.sub.3 /N.sub.2 they find that both fluorine and nitrogen play a role in etching silicon. They note that higher powers are needed to sustain the plasma of this type of mixture than for NF.sub.3 /Ar. The results also indicate that the N-F radical may also etch silicon and result in a higher etch rate than with NF.sub.3 /Ar. Again they observe a nonlinear relationship between etch rate and NF.sub.3 concentration at constant power.
Low etch rates were obtained for the NF.sub.3 /H.sub.2 mixes. They explain this by noting that hydrogen scavenges fluorine. Additionally, material deposition was observed for R=0.4.
The prior art has attempted to use nitrogen trifluoride for various etching processes, but the prior art has not resolved the problem of efficient use of nitrogen trifluoride as the present invention has done in which lower requirements of expensive nitrogen trifluoride at higher etch rates result in shorter down time for semiconductor process equipment, more thorough etch-cleaning with lower utilization of nitrogen trifluoride as will be set forth in greater detail below.